While visiting a health studio in Beverly Hills, fashion model Jennifer Downing, the daughter of millionaire P.J. Downing, is kidnapped. Her father turns to a family friend, Eliot Draisen, who is president of the detective agency Crumb & Crumb, to investigate the case.

Eliot is reluctant to supply P.J. with one of his capable detectives because, as it turns out, Eliot himself is the organizer of the kidnapping. But to give the appearance of taking the investigation seriously, Eliot offers P.J. the services of Harry Crumb (Candy), the last descendant of the agency's founders. Eliot talks up Harry's detecting skills to persuade P.J. that he's the right man for the job, but he secretly knows that Harry is incompetent and counts on this fact to allow him to slip right through the detective's fingers.

Harry returns to Los Angeles (by bus) from an assignment in the firm's Tulsa branch office (which he messed up, as usual). He immediately begins his investigation, assisted by P.J.'s younger daughter, Nikki, who is considerably smarter than he is. Harry's tendency to mix fact with fantasy, however, does not prevent him from noticing that Nikki's stepmother, Helen Downing, is having an affair with her tennis coach Vince Barnes, and after several incidents and observations, the detective comes to the conclusion that she is behind the kidnapping. Helen is also the object of affection of Eliot Draisen, but all she is interested in is money; to this end, she tries to get rid of her husband on several occasions and even does her best – along with Barnes – to get the ransom for herself.

Despite several close calls, Eliot manages to outwit Harry and the police. He escapes to the airport, from where he intends to depart to Buenos Aires. But he makes the mistake of informing Helen of his plans; she and Barnes take the money and leave him, tied up and gagged, in a utility room at the airport. Harry and Nikki manage to trail the kidnapper to the airport, where the detective corners Helen and Barnes just as their plane prepares to take off. After Jennifer is freed, Harry and the police find Eliot in the side room. Fed up with Harry's dumb luck – and owing to a slight misunderstanding – Eliot confesses and is sent to prison.

With the Downings' gratitude boosting his confidence, Harry takes over as the new president of his family's business, where a new case immediately piques his personal attention.